Turbidity meters have to be calibrated regularly. To this end, calibration suspensions containing a turbidity calibration standard in a pre-determined concentration are often used. The calibration suspensions are made by the manufacturers themselves or obtained commercially as a finished standard suspension in canisters of various volumes. For calibration, the standard suspension is optionally diluted and introduced into the turbidity meter. Subsequently, light intensity is measured and correlated with the concentration of the turbidity calibration standard in the standard suspension.
The calibration suspensions usually comprise colloidal particles in a suitable liquid. The internationally recognized turbidity calibration standard is formazine. The manufacture of this calibration standard is described in the ISO 7027 standard.
Over time, the colloidal particles settle on the ground of the storage container. To calibrate a turbidity meter, the calibration suspensions have thus to be mixed before use. This is done by shaking or stirring of the calibration suspension in the storage container.